Simon Cowell and Lauren Silverman attend the ITV Gala at London Palladium in November 2015 C. Wilson/Getty Images

A security guard has spoken of the dramatic moment he grappled with a burglar to rescue a bag containing an estimated £500,000 of jewellery belonging to X Factor boss Simon Cowell. M'Bathonha Mendes, 34, revealed that he was sitting in a car outside the pop mogul's £35m mansion in Holland Park, West London, when he spotted a suspicious man walking along the street.

Portugal-born Mendes, who has since been branded a "hero"' by Cowell, said the individual matched the description of a man who had recently burgled a nearby property and he decided to investigate further when he saw an item fall out of his bag. Once he realised the item was Cowell's passport, Mendes gave chase, grabbing the man by the shoulder and ordering him to wait.

"The adrenaline just kicked in. He was trying to get something out of the bag and I ran up to him, put my hand on his shoulder and said, 'wait a second'," Mendes told the Sunday Mirror. "He jumped and then dropped the bag and ran. Fear is my friend," he added.

The newspaper quoted a security source as saying the rucksack's contents were worth £500,000.

Simon Cowell's house in Holland Park, LondonGoogle Maps

Cowell said he and his family had been left traumatised by the burglary, which took place in the early hours of Friday morning and lasted for half an hour. But he heaped praise on Mendes – branding him a hero and revealing he's line for a large Christmas bonus.

"The most frightening thing is someone has been in your house. It's not what they steal, it's that's someone can break into your house. It's nerve-racking," Cowell told The Sun.

Cowell is understood to have sacked his own personal security officer who was unaware of the break-in, because he was in the toilet. When asked about his own employee, Cowell said: "One person won't be here anymore."

Speaking of his own reaction to the burglary, Mendes said: "I haven't spoken to Simon but he has phoned my boss Trevor and wants to meet me. I am really glad he thought I did a good job. My daughter Sara is a huge X Factor fan and can't believe I saved some of Simon Cowell's stuff."

Worst guard dogs ever

Cowell has also revealed that he feared for the life of his young son Eric, adding that he and girlfriend Lauren Silverman rushed to check on the 21-month-old's safety once they had been informed of the break-in.

"That was the first thing we did, to make sure he was OK," he told The Sun. "Apart from the fact someone broke into my house and stole my stuff, which is scary, it could have been worse. Thankfully no one was hurt," he added.

Cowell did manage to find some humour in the situation, however, joking that he had "the worst guard dogs in the world"' after his Yorkshire terriers Squiddly and Diddly failed to alert them to the burglar's presence. "Maybe we'll get bigger dogs," he added.

He also referenced the break-in during last night's episode of the X Factor, joking that Reggie 'N' Bollie's version of "Locked Away" by R City featuring Adam Levine was "very appropriate the night after I get burgled".